Method and device for treating natural gas containing water and condensible hydrocarbons

ABSTRACT

A method of treating gas containing water in order to remove at least part of the water from the gas, including feeding the natural gas to be treated by a first line, with a liquid fraction containing at least an aqueous phase is fed via a second line in the presence of a solvent into a contact zone, so as to bring the gas into direct contact with the liquid fraction over at least a portion of the contact zone. The solvent is a non-hydrocarbon compound other than water, and simultaneously, the gas is cooled in the presence of the solvent in order to condense at least one liquid phase consisting essentially of water in a mixture with the solvent. The non-condensed gaseous phase is separated from the condensed liquid phase, from which the solvent has essentially been removed

FIELD OF THE INVENTION

The present invention relates a method of treating a gas, in particulara method of dehydration, and a plant for implementing the method.

The present invention has particular advantages when applied to thedehydration of natural gas.

Advantageously, it can be used to separate condensible hydrocarbons fromnatural gas, for example those which have at least three carbon atoms.

BACKGROUND OF THE INVENTION

Petroleum products and natural gas in particular contain products whichare undesirable for the purposes of transportation and/or handling.

Of these substances, one of the main constituents to be removed iswater, which has proved to encourage the formation of hydrates and isconducive to corrosion. In practice, hydrates can cause transportpipelines to become clogged and blocked, preventing gas from passingthrough them in the long term, and the corrosive action of the gascauses deterioration in pipelines and processing installations. Thesetwo factors are extremely detrimental in terms of their consequences andcan often lead to quite lengthy production stoppages because it is verydifficult to break down any hydrates which have formed, incurring severefinancial losses.

Various methods have been described in the prior art as a means ofovercoming these disadvantages.

Patent FR 2.605.241 describes a method of treating natural gas spanningseveral stages performed in several devices in succession. In a firstenclosure, the gas to be treated is brought into contact with a cooledphysical solvent in order to produce a gas which is saturated with theadded water from the solvent and this gas is then cooled in an exchangerin order to condense the aqueous phase containing the solvent and thesaturation water as well as a phase of liquid hydrocarbons. The cooled,dehydrated gas and the fraction containing the condensed hydrocarbonsare then separated in a separator unit.

This method has significant advantages as compared with the techniquesused in the art. However, the methane and hydrocarbons such as propaneas well as hydrocarbons with more than three carbon atoms are not fullyseparated.

In addition, it is necessary to use at least two devices, which requiresadditional space in the processing plants incurring the additionalcapital investment.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art andprovides a less expensive method of treating gases, in particularnatural gas, which contain water in different forms and need to bedehydrated. It can also be applied in the treatment of gas produced inindustrial plants, for example refinery gases. The method and plant ofthe invention require only one enclosure for the dehydration process,for example an exchanger column, which means that the installation isless expensive than plants known from the prior art and requires lessspace.

It can be applied to advantage in the case of natural gas containing atleast water, methane and condensible hydrocarbons such as the C₃₊ and/orpossibly the C₂₊.

Throughout this description, C₃₊ will be used to denote hydrocarbonscontaining at least three carbon atoms per molecule, C₄₊ to denote allhydrocarbons having at least four carbons atoms and C₅₊ all hydrocarbonshaving at least five carbon atoms.

The expression "treated gas" will be used to denote dehydrated gas orgas from which at least part of the saturation water has been removed.

The present invention relates to a method of treating a gas containingat least water, the purpose being to remove the water at least partiallyfrom the said gas.

It is characterised in that it consists of at least the following steps:

a) the natural gas to be treated is delivered by means of a first lineand a liquid fraction containing at least an aqueous phase is deliveredby means of a passage in the presence of a solvent into a contact area,for example, in order to bring the said gas and said liquid fractioninto direct contact over at least a part of the contact area, the saidsolvent being a non-hydrocarbon substance other than water, and the saidgas is at least partially cooled in the presence of the said solvent inorder to condense at least a liquid phase essentially consisting of awater in mixture with the solvent, and

b) the non-condensed gaseous phase is separated from the condensedliquid phase from which the solvent has been at least partially removed.

The treated gas is preferably circulated in a rising flow whilst thecondensed liquid phase flows downwards.

Advantageously, the liquid phase containing the solvent is fed in fromthe head of the contact area of the gas and liquid fraction.

In a preferred embodiment of the method of the invention, the gas to betreated flows upwards whilst the liquid phase containing the solventcirculates in counter-flow.

The gas to be treated is cooled by producing a temperature gradient, forexample, which will vary depending on the nature of the said gas and/orthe said solvent.

Another embodiment uses a process of self-cooling where the gas iscooled using at least a part of the treated gas.

The quantity of solvent phase to be injected in is determined on thebasis of the temperature and/or temperature gradient measured during theprocess, for example, and the nature of the components of the said phasein conjunction with the operating conditions, such as temperature.

The solvent may be an alcohol or a solvent selected from among thefollowing products: dimethoxymethane, dimethoxyethane, ethanol,methoxyethanol, propanol or may possibly be selected from the differentclasses of solvents such as the amines or ketones, for example.

It as an advantage to use methanol as the solvent.

A gas containing at least water and constituents is other than waterwhich are condensible at different temperatures is put through steps a)and b), whereby the gas is brought into contact with a liquid fractioncontaining an aqueous phase and a phase containing the said condensibleconstituents during step a), for example, and the said liquid fractionfrom which the solvent has been at least partially removed is separatedinto an aqueous fraction and one or more phases containing the saidcondensible constituents.

The method is particularly well suited to the treatment of natural gascontaining at least methane, water and hydrocarbons higher than C₂ toC₅, where the treated gas, water and/or the LPG fraction and/or gasolineare separated.

The present invention also relates to a plant for treating a gascontaining at least water, the purpose being to remove at least part ofthe water.

It is characterised in that it comprises an enclosure EC₁ with andelivery line for the said gas to be treated and at least one deliveryline for at least one liquid phase containing at least one solvent, eachof the said lines linking in to a main circuit which will allow the saidgas and the said solvent phase to be brought into contact. It also hasat least one discharge line for the treated gas and at least onedischarge line for a liquid phase, the said discharge lines being linkedto the said main contact circuit and a cooling circuit which isthermally linked to the main contact circuit. The delivery line for theliquid phase containing the solvent is located at the upper part of thedisclosure.

The gas and the liquid fraction are brought into contact with oneanother in the presence of a solvent over at least a portion of thelength of the main circuit.

In one embodiment, the plant is adapted to treat natural gas containingat least water and components which condense at different temperaturesand are required to be recovered selectively.

For this purpose, the installation has at least one means for recoveringthe said condensed constituents and at least one outlet linecommunicating with the said recovery means.

The plant may have means for stabilising the said condensed constituentslinked to one of the said discharge pipes for the condensed constituentsand/or integrated within the enclosure, by preference in its lower part.

Advantageously, the method and plant of the invention are used todehydrate a natural gas containing water and at least one hydrocarbonother than methane and to separate at least some of the hydrocarbonsother than methane.

The plant of the invention offers the specific advantage of beingcompact and less expensive and optimises the process of recoveringcertain hydrocarbon fractions.

The implementing method reduces the cost of treating natural gas andincreases output in terms of the selected hydrocarbon fractions.

Other features and advantages of the invention will become clearer fromthe following description giving examples of different applicationsinvolving the treatment of natural gas, which are not restrictive, andwith reference to the appended drawings, in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of the basic principle of theinvention using an external cooling circuit,

FIGS. 2A and 2B describe an installation having two cooling circuits, anexternal circuit and a heat exchange circuit using at least some of thetreated gas, as well as stabilising means,

FIG. 3 shows a variant of the self-cooling system in which the treatedgas is recycled and used as coolant,

FIG. 4 illustrates a method in which an installation has means forrecovering the liquid hydrocarbons from a natural gas,

FIGS. 5 and 6 illustrate plants integrating means for drawing offdifferent condensates linked to the stabilising means,

FIG. 7 is an example of a plant which integrates an optimisedstabilisation process,

FIGS. 8 and 9 illustrate variants of the method in which stabilisationoccurs in a heat exchange zone,

FIGS. 10, 11 and 12 are examples of the technology used to make the heatexchanger, and

FIG. 13 illustrates an example of the technology used for the heatexchanger.

DETAILED DESCRIPTION OF THE INVENTION

The implementing method of the invention consists in simultaneouslycooling a gas containing saturation water and bringing it into contactwith a liquid fraction in the presence of a solvent, the purpose ofwhich is to prevent the formation of ice and/or hydrates.

The cooling stage causes the saturation water contained in the gas aswell as the liquid hydrocarbons contained in a rich natural gas to becondensed.

Advantageously, these two steps are effected in a single enclosureincorporating at least one main or contact circuit for the gas, theliquid phase and the solvent and a cooling circuit.

In the case of a rich natural gas, the plant has an advantage in that itcan be used to fractionate and recover the liquid hydrocarbons forvarious compounds at different temperatures depending on the compositionof the treated gas and the requirements of the producer.

The principle of the method is illustrated in FIG. 1 and is applied, byway of example, to a water-saturated natural gas containing theassociated high hydrocarbons.

The gas to be treated is fed into an enclosure EC₁ such as a heatexchanger via a line 2 located in its lower part. Inside this exchangerEC₁, it is simultaneously brought into contact with a liquid phase madeup at least partially of a solvent fed into the exchanger EC₁ via theline 3 and cooled by a process of indirect heat exchange, across a wall(FIGS. 8, 9), for example.

A heat-exchange medium can be used for the cooling process and willpenetrate the shell of the exchanger EC₁ from the line 4 circulatingfrom the top down to reduce the temperature of the gas to be treated asit rises, before being fed out of the exchanger via the line 5.

By preference, the gas to be treated is brought into contact incounter-flow and continuously with the liquid phase containing thesolvent, which moves down inside the exchanger EC₁ as the gas to betreated moves up. The gas is preferably cooled by a continuous processof heat exchange in counter-flow.

This cooling process causes the heavy hydrocarbons contained in the gasand a part of the saturation water in the gas to condense. These twocondensed liquid phases flow downwards through the device by force ofgravity, in counter-flow with the treated gas, which gradually losespropane, butane and heavier hydrocarbons as a result of the exchange ofmatter between the gaseous phase and the liquid hydrocarbons. Thecondensed liquid hydrocarbon phase gradually becomes enriched withheavier constituents as it moves downward. The condensed aqueous phaserich in solvent at the head of the exchanger loses solvent as it comesinto contact with the gas.

These two phases are separated by a process of decantation in the lowerpart of the exchanger column before being discharged respectively viapassages 7 and 8.

It is also possible to discharge all of these two phases through asingle passage located in the lower part of the exchanger, in theposition of the line 7, for example.

The treated gas charged with solvent is discharged from the head of theexchanger column EC₁ through the line 6.

The gaseous phase charged with solvent and the condensed liquid phaseare treated separately, for example, depending on their subsequent useor mode of transportation or to meet the specifications of the produceror consumer.

The evaporated solvent entrained with the gaseous phase will prevent theproblems of hydrate formation associated with cooling.

The exchanger EC₁ may also be provided with means 9 to control thetemperature, such as temperature sensors for example, which will beconnected to control and processing means 10. The temperature sensorsmay be distributed along the heat exchanger EC₁ to measure theprevailing temperature at several points along the path of thecirculating gas.

Before it is fed into the exchanger EC₁, the gas to be treated may becooled using whatever coolant is available, such as water and/or air, inan exchanger E₁ located on the line 2. Under the temperature andpressure conditions at the outlet of the exchanger E₁, this firstcooling step will separate a gasoline fraction made up of thecondensible hydrocarbons.

A solvent is used which is at least partially miscible with water. Bypreference, its boiling temperature will be lower than that of water sothat it can be entrained by the non-condensed gas.

This solvent may be an alcohol, for example, preferably methanol. It mayalso be selected from the following group of solvents:methylpropylether, ethylpropylether, dipropylether,methyltertiobutylether, dimethoxymethane, dimethoxyethane, ethanol,methoxyethanol, propanol it or may be selected from different classes ofsolvents such as the amines or ketones, for example, or possibly amixture made up of one or more of these products.

The quantity of solvent to be injected is usually adjusted to suit thetemperature and pressure and/or the composition of the gas in order toavoid the formation of hydrates and the formation of ice crystals due tothe presence of the water.

For example, the ratio by mole of the flow rate of the solvent to theflow rate of the treated gas is within the range between 1/1000 and1/10.

Advantageously, the treatment process can be optimised by adjusting thequantity of solvent injected depending on the composition of the gas,for example, and/or the operating conditions such as the temperatureand/or the temperature variation and/or the pressure. To this end,account is taken of the temperature values and/or the temperaturegradient measured by the temperature sensors located on a level with theexchanger.

By preference, account is also taken of any processing to which the gaswill be subjected once it leaves the enclosure.

Since it circulates in counter-flow, the gas entrains with it thesolvent contained in the liquid phases which move downwards by force ofgravity. These liquid phases are collected at the base, the solventhaving been essentially been removed from them. The solvent injected atthe head is therefore mainly discharged in the gaseous phase leavingfrom the head. The quantity of solvent to be injected can therefore beadjusted to produce the level of concentration in this gaseous phaserequired to prevent hydrate formation, taking account of the temperatureand pressure conditions.

The lower the temperature, the lower and weaker the solvent content inthe gaseous phase will be. The quantity of solvent injected from thehead of the exchanger will therefore be relatively low and constitutes asupplement intended to compensate the losses in the gas.

The solvent injected from the head does not need to be pure and may bemixed with water, for example, provided however that the concentrationof solvent in aqueous phase will prevent hydrate formation.

The variation in temperature or the temperature gradient induced in theexchanger are selected on the basis of the nature of the gas and thequantity of condensed hydrocarbons, such as the LPG and gasoline, to berecovered.

Similarly, the temperature of the gas to be treated is preferablyreduced in order to obtain a temperature gradient across the wholeexchanger.

Advantageously, the method is partially or fully self-cooling, i.e. atleast a part of the treated gas is used for the cooling process.

FIG. 2A illustrates an embodiment in which the heat exchanger EC₁ isprovided with a cooling circuit which uses a heat-exchange fluid,entering via the line 4 and leaving via the line 5 illustrated in FIG.1, lines 4 and 5 being located in the upper part, and an additionalcooling circuit is provided which uses at least some of the treated gasdischarged from the exchanger via the passage 6, which enters theexchanger via a line 11 preferably located below the discharge line 5for the heat-exchange medium, and leaves via a line 6' on a level withthe lower part of the exchanger.

The external heat-exchange fluid cools at least some of the treated gasin the upper part of the exchanger, which is discharged via the line 6and fed back into the exchanger via the line 11, forming an extension tothe line 6.

A liquid fraction containing the greater part of the heavier componentscontained in the gas fed into the heat exchanger EC₁ and condensedduring the process is discharged via the line 8.

This liquid fraction can be stabilised by heating the volume of liquidcollected at the base of the heat exchanger EC₁ as illustrated in FIG.2B, where a stabilising means such as a reboiler B₁ is incorporated. Aliquid fraction which has already been stabilised is therefore collectedvia the line 8 and the output of C₁ -C₂ is improved, since the methaneand ethane evaporated by the heating process are contained in thetreated gas which is discharged through line 6'.

EXAMPLE 1

A first example of how the method of the invention is implemented isillustrated in FIG. 2A, whereby a natural gas containing the associatedhigh hydrocarbons is treated to produce a dehydrated gas from which theconstituents containing at least three carbon atoms have largely beenremoved.

The heat exchanger EC₁ is a tubular column-exchanger, for example, thetubes being lined to increase the transfer of substances between thegas, the aqueous phase, the hydrocarbon phase and the solvent.

The composition of the natural gas by mass is, for example as follows:

    ______________________________________    WATER            82.30    METHANOL         0.00    NITROGEN         211.97    CARBON DIOXIDE   397.79    METHANE          25765.00    ETHANE           6028.62    PROPANE          4360.50    ISOBUTANE        1335.05    BUTANE           487.21    ISOPENTANE       668.81    PENTANE          528.87    HEXANE           1053.47    TOTAL KG/HR      42919.59    ______________________________________

The natural gas is firstly cooled in the exchanger E1, for example, to atemperature greater than or equal to its dew point close to 43° C. and apressure essentially equal to 4.4 MPa before being delivered to thetreatment device via the line 2 at a flow rate essentially equal to42919 Kg/h. The solvent, essentially consisting of methanol, is injectedvia line 3 at a temperature lower than or equal to the ambienttemperature and, for example, close to -20° C., at a flow rateessentially equal to 13 Kg/h.

The upper part of the exchanger EC₁ is cooled by a heat-exchange mediumdelivered via line 4 at a temperature close to -27° C. and which isdischarged via line 5 at a temperature close to -23° C.

The lower part of the exchanger EC₁ is cooled by the treated gasdischarged via line 6 and fed back into the exchanger via line 11 at atemperature essentially equal to -22° C. and at a pressure close to 4.4MPa. The recycled treated gas lowers the temperature of the gas to betreated as it circulates in counter-flow by a process of heat exchangebefore it is discharged via line 6' at a temperature of 39° C.

As it cools, the natural gas to be treated gradually loses on the onehand hydrocarbons containing more than three carbon atoms which condenseto form a liquid phase made up of liquid hydrocarbons, and, on theother, water, the aqueous phase formed by condensation of the saturationwater being gradually discharged as it is formed.

During the dehydration process, the natural gas, the methanol and thetwo liquid phases are in contact. The presence of the methanol preventsthe formation of ice and hydrates.

If the quantity of methanol delivered at the head is limited to theamount needed to prevent hydrate formation, the condensation water fromthe treated gas collected at the base of the exchanger EC₁ contains lessthan 100 ppm of solvent. This water is evacuated via the line 7 at atemperature of 43° C. at a flow rate close to 82.0 Kg/h. The liquidhydrocarbon phase enriched with C₃₊ hydrocarbons and containing morethan 99% C₅₊ hydrocarbons, 65% of the C₄₊ components and 12% of C₃₊, forexample, is discharged via the line 8 after being separated from theaqueous phase, by a process of decantation for example, at a flow rateof about 5175 Kg/h.

The treated gas, dehydrated and having lost some 50% of its C₃₊hydrocarbons, is discharged via the line 6 at a flow rate of 37710 Kg/h.The production yield of C₁ and C₂ is about 98% whereas the yield wouldbe in the region of 92% if a method of the prior art were used. Inanother embodiment, the plant is self-cooling since it uses some of thetreated gas from line 6 as the coolant.

An example of this type is illustrated in FIG. 3. The treated gasleaving the head of the exchanger is cooled in a device 12 located onthe line 6, by being expanded through a valve, for example, or by beingexpanded through a turbine. Having been cooled in this way, the gas isdelivered through line 4 back into the exchanger EC₁, where it reducesthe temperature of the gas to be treated flowing upwards by a process ofheat exchange. After the heat exchange, it is fed out through line 6'before being sent to another processing plant and/or a transportpipeline.

The gas discharged via line 6' may be sent to a compression device (notillustrated) downstream of the exchanger for transportation.

Advantageously, the plant can be used to recover the cuts of heavierhydrocarbons from a natural gas, depending on their composition, inparticular the number of carbon atoms per molecule.

FIG. 4 illustrates a treatment plant incorporating means for recoveringthe LPG fractions from a natural gas and separate means for recoveringgasoline.

The embodiment illustrated in FIG. 4 differs from that of FIG. 2 in thatit has recovery means (14, 15) arranged on a level with the exchanger.This embodiment could therefore also be incorporated with the devicesillustrated in FIGS. 1 and 3 without departing from the scope of theinvention.

Natural gas contains hydrocarbons which condense at differenttemperatures. Reducing the temperature on a given gradient inside theexchanger will allow the different hydrocarbon fractions contained inthe natural gas to condense in different zones. The heavier fractionswill be collected at the bottom of the exchanger and the lighterfractions at the head of the exchanger. A hydrocarbon fraction with afixed boiling point within a certain range could also be collected froman intermediate zone.

In order to recover separately the LPG fraction, for example, whichcontains propane and the butanes (hydrocarbons with three or four carbonatoms) and the gasoline representing the C₅₊ fraction, the exchanger EC₁is fitted with a recovery means, such as a plate 14, communicating withthe main zone in which the treated gas and solvent are brought intocontact. The recovery plate 14 is located in a part of the exchanger,the level of which is fixed as a function of the nature of cuts orhydrocarbons to be recovered and the temperature prevailing at differentpoints of the column.

The plate 14 will allow the condensed aqueous phase to be separated fromthe condensed hydrocarbons chiefly containing the propane and butanes aswell as a small quantity of methane and ethane and collect them so thatat least some of them can be discharged via a lateral line 15.

The separated aqueous phase as well as the hydrocarbon phase at theplate 14 which was not recovered move from the plate 14 towards the mainprocessing circuit where they continue their downward movement beforebeing discharged via lines 7 and 8 as described in connection with FIG.1, for example.

Clearly, the exchanger may be fitted with several recovery plates linkedto lateral discharge lines distributed along the exchanger depending onthe nature of the hydrocarbons to be recovered.

In certain cases, it is an advantage to recover the solvent from a stagein which the LPG are washed externally to the exchanger by a process ofliquid-liquid extraction using a "washing" and/or quantities of solventfrom other devices in the processing plant.

Advantageously, the plate 14 has at least one passage 144 through whicha liquid phase such as an aqueous phase containing a solvent can bedelivered.

The liquid phase delivered via the line 144 may come from a washingstage of a liquid fraction drawn off, such as the liquid hydrocarbonfraction drawn off via the line 15, for example. At the level of theplate 14, this fraction of liquid hydrocarbons is in fact in equilibriumwith an aqueous phase partly containing solvent and the gaseous phasewhich also contains solvent. As a result of this equilibrium, the liquidhydrocarbons drawn off via line 15 contain traces of solvent.

The aqueous phase from the washing stage which contains the solventinitially dissolved in the liquid hydrocarbon fraction is recycled tothe exchanger column EC₁ via the line 144 and then brought into contactwith the gas to be treated in counter-flow. During this contact process,it undergoes an exchange of matter giving off the quantity of solventwhich is entrained in the gaseous phase.

The line used to re-inject the liquid phase containing the solvent to berecovered may also be located at other levels of the exchanger column.Several injection lines of this type may also be provided on theexchanger column depending on what processing is required.

Recovering this solvent and any other dissolved products will reduceoperating costs.

These embodiments improve the recovery of the components of a condensatefor different types of gas.

The can also be applied with gases other than natural gas, for examplewith refinery gases.

EXAMPLE 2

A second example of the way in which the invention is implemented isdescribed in conjunction with FIG. 4 where the cuts of hydrocarbonscontained in the natural gas can be recovered on the basis of a desiredcomposition, for example the relevant hydrocarbons containing more thanthree carbon atoms.

The natural gas is cooled and fed through the contact process describedin FIG. 3, for example. The other methods described in FIGS. 1 and 2 mayalso be applied without departing from the scope of the invention.

During the dehydration process, the natural gas loses its heavy C₃₊constituents by a process in which some of the components made up ofmolecules containing at least three carbon atoms condense to form aliquid hydrocarbon phase which is enriched with increasingly heavycomponents from the top down.

If the dimensions of an exchanger column EC₁ are such as to obtain theequivalent of 12 theoretical plates, the concentration of propane in theliquid hydrocarbon phase obtained at the level of the sixth plate isclose to 26% by mass, for example, whereas it is 9.8% in the liquidhydrocarbon phase discharged from the base of the system via line 8.Since the concentration of propane builds up along the exchanger column,it is desirable and an advantage to draw off the hydrocarbon phasesformed at the levels which will produce the required compositions.

For example, lateral branches can be used to draw off liquid hydrocarbonphases of variable composition to suit the producer's specification.

Natural gas is fed in via the line 2 at a flow rate of 42837 Kg/h, atemperature close to 43° C. and a pressure substantially equal to 4.5MPa. Methanol is delivered via the line 3 in a quantity of 13.9 Kg/h tomaintain the concentration of methanol in the aqueous phase formed inthe exchanger at a gradient corresponding to a concentration whichvaries between 99% mass at the head of the exchanger at a temperature of-23° C. and a concentration of 0.01% by mass at the base of theexchanger at a temperature of approximately 43° C.

On a level with the recovery means 14 and 15 (FIG. 4) corresponding tothe fourth theoretical plate level of the exchanger, 70% of the liquidhydrocarbon phase containing more than 25% by mass of propane isdischarged via the line 15 at a temperature of about -2° C. and at aflow rate essentially equal to 3600 Kg/h.

The aqueous phase and the hydrocarbon phase containing the majority ofthe carbon atoms greater than C₄₊ are discharged after decantation viathe lines 7 and 8.

The liquid hydrocarbon phase containing the C₅₊ hydrocarbons , forexample, which is discharged via the line 8 at a flow rate of 2525 Kg/h,contains more than 75% of the C₃₊ hydrocarbons contained in the gas tobe treated.

The treated natural gas cooled by expansion through the device 12 isused in the heat exchange process to cool the natural gas to be treatedcirculating in counter-flow before being discharged after the exchangeprocess via the line 6' at a flow rate of 36715 Kg/h. It has beendehydrated and more than 55% of its C₃₊ hydrocarbons have been removed.

The advantage of recovering the hydrocarbons from selected zones isevident when the two examples described in relation to FIGS. 2 and 4 arecompared.

A hydrocarbon phase enriched by 65% with C₃ and C₄ components isrecovered on a level with the plate 14, whereas the hydrocarbon phasedischarged via the line 8 is enriched by only about 20% in C₃ and C₄.

On the other hand, if cuts are recovered on lateral branches on thebasis of their composition and their condensation zone, the liquidhydrocarbon phases obtained are different in composition, one rich in C₃and C₄ components drawn off on a lateral branch and the other rich in C₅and C₆ discharged from the base of the system.

This method will also increase the yield of C₁ and C₂ components fromthe treated gas.

Advantageously, the plant may also be used to stabilise the hydrocarbonfractions recovered using one of the methods described in relation toFIGS. 1 to 4.

FIGS. 5 and 6 respectively illustrate plants which incorporate adehydration device provided with means for stabilising the condensedhydrocarbons (LPG and gasoline).

In FIG. 5, the discharge line 15 communicating with the recovery plate14 for the condensed LPG of FIG. 4 is connected to a device 16 whichwill allow these to be stabilised. The device used is a stabilisingdevice known to the skilled person, for example, and will therefore notbe described.

In another embodiment, the stabilising device used is an exchanger suchas that previously described, whereby an exchange of heat and anexchange of matter occur simultaneously, as will be described below inrelation to FIGS. 8 and 9.

The stabilisation method incorporated in the process used to recover thehydrocarbon fractions consists in feeding the condensate fractionrecovered from the level of plate 14, which contains a small quantity ofmethane and ethane and a majority of condensed LPG, into the stabilisingdevice 16.

The fraction rich in methane and ethane is discharged from thestabilising device via a line 17 and recycled to the exchanger EC₁ on alevel with plate 14 to be collected and mixed with the gas to betreated.

The stabilised LPG fraction is fed out from the lower part of thestabilising device on a level with the reboiler 19 via a passage 18.

This approach has the advantage of enabling the LPG to be stabilisedbefore they are recovered by the producer and of increasing the methaneand ethane output of the method.

In FIG. 6, the plant described in FIG. 5 has a second stabilising device21 which is used to stabilise the gasoline discharged via the line 8.

The operating diagram is identical to the one described in relation toFIG. 5 where the condensate containing mostly the gasoline and a smallquantity of C₁ to C₄ hydrocarbons is discharged via the line 8 and fedto the stabilising device 21.

The stabilised gasoline is discharged through the line 22 on a levelwith the reboiler 23.

The gaseous fraction mainly made up of methane, ethane and propane isevacuated from the device via the line 25 to be recycled and re-mixedwith the gas to be treated as it arrives via the line 2.

These two embodiments provide optimum recovery of the hydrocarbonfractions.

The stabilisation process of the LPG fractions and gasoline produced canbe improved.

For this purpose, the plant described in FIG. 7 differs from that ofFIG. 6 by dint of two expansion valves V₁ and V₂ located respectively onthe discharge lines 14 and 8.

The gaseous fractions from the stabilising devices 16 and 21 arere-compressed in means such as compressors K₁ and K₂ before being fedback via a line 28 into the gas to be treated on a level with thepassage 2.

Different technologies, known to the person skilled in the art, can beused to set up the exchange process and the relevant means or devices,some of which will be described below by way of example, are notrestrictive.

FIGS. 8 and 9 illustrate variants of the embodiment of the invention inwhich a liquid fraction is stabilised in a contact and heat exchangezone such as the one described above.

To this end, at least one of the liquid fractions collected is fedthrough a contact and heat exchange zone where it is simultaneously :

brought into contact, in counter-flow, with a rising vapour phase,

heated by a process of indirect heat exchange in the contact andexchange zone.

In the case of two contact and heat exchange zones Z₁ and Z₂, forexample, the liquid fraction from zone Z₁ can be stabilised in a contactand exchange zone Z₃ as illustrated in FIG. 8.

In this example, the gas to be treated arrives at the contact zone Z₁via the line 50. A gaseous phase enriched with solvent is collected fromthe head of this contact and heat exchange zone Z₁ and fed to thecontact and heat exchange zone Z₂ and then leaves the head of thiscontact zone Z₂ via the line 52. A relatively light liquid fraction isrecovered, which is discharged through the line 53.

At the head of zone Z₁, a solvent is fed in via the line 100, to preventthe formation of hydrates for example, most of this solvent beingevacuated with the gas leaving via the line 52.

A relatively heavy liquid fraction is collected at the base of thecontact and exchange zone Z₁ and is fed out via the line 54.

This liquid fraction is then delivered to the contact and heat exchangezone Z₃, where it is simultaneously:

brought into contact with the rising vapour phase,

heated by a process of indirect heat exchange. This indirect exchange ofheat may be between an external heating fluid arriving via the line 51and leaving via the line 55 on the one hand and on the other the liquidfraction, which is picked up by the pump P1 as it leaves zone Z₃ via theline 56 and fed to zone Z₃, from which it is discharged cooled via theline 57. The external heating fluid may be the gas to be treated, forexample, if it is hot enough, such as when it leaves a compression stagefor example, or any other fluid that is available at an adequatetemperature.

after it has passed through the contact and treatment zone, the liquidfraction is discharged in a stabilised form, i.e. the lighterhydrocarbons which it contained have essentially been removed, on alevel with the lower part of the exchange and reaction zone Z₃.

A liquid fraction from a contact and exchange zone can also bestabilised by feeding it in downward flow into a contact and heatexchange zone located below the contact and-exchange zone from which theliquid fraction has come, in counter-flow with the vapour phasegenerated by heating in the contact and exchange zone in which thestabilisation occurs.

In the case of two contact and exchange zones Z₁, Z₂, for example, theliquid fraction from zone Z₁ can be stabilised in zone Z₂ as illustratedin the diagram of FIG. 7.

In this example, the liquid fraction from zone Z₂ is fed to zone Z₁where it circulates in counter-flow with the vapour phase generated byheating in this zone Z₁. This exchange of heat also helps to cool thegas arriving via the line 50. The stabilised liquid fraction isevacuated via the line 60. The vapour fraction from this stabilisationstage is discharged from the head of zone Z₁ and fed from 61 into zoneZ₂.

The liquid fractions from one of the contact and heat exchange zones canalso be stabilised in a contact and heat exchange zone operated at ahigher temperature once they have been expanded to facilitate thestabilisation process.

In this case, the vapour fractions from such a stabilisation stage mustbe re-compressed before being fed into a higher contact and exchangezone.

The method of the invention can be used to separate, fractionate andstabilise the condensible fractions contained in a gas to be treated.

In the case of a natural gas, for example, and using three contact andheat exchange zones Z₁, Z₂ and Z₃ operating as illustrated in thediagram illustrated in FIG. 6, it is possible to produce a stabilisedgasoline fraction with C₅₊ at the base of zone Z₃ and a LPG fraction atthe base of zone Z₂, which can be stabilised in zone Z₁ in line with thediagram illustrated in FIG. 7.

By providing a fourth exchange and contact zone Z₁ Z₂ located above thezone Z₁,Z₂ and operating at a lower temperature, it is also possible toseparate a C₂ fraction which can be stabilised by heat exchange byfeeding it through zones Z₁ and/or Z₂, for example.

The exchanger EC₁ is a shell and tube heat-exchanger, for example, suchas that illustrated in FIG. 10.

The gas to be treated arrives via the line 2 and flows upwards insidethe vertical tubes 30. These tubes are preferably provided with alining, such as a structured lining, so as to improve the contactbetween the rising gas and the liquid fractions flowing downwards. Thetreated gas is discharged from the head via the line 6. The solvent isfed in from the line 3 and directed into the different tubes 30 by meansof a feeder ramp 31 towards a distribution plate 32.

The liquid hydrocarbon phase, stabilised through heat by means of areboiler B₂ located in the lower part of the exchanger EC₁, for example,is evacuated at a controlled level via the line 8 and the aqueous phaseis discharged at a controlled level via the line 7.

A heat exchange medium is used for cooling and is fed into the exchangervia the line 33 and discharged after the heat exchange via the line 34.

Using a different technology, the heat exchanger EC₁ could be a plateexchanger, made from brazed aluminium for example, such as thatillustrated in FIG. 11.

A heat exchanger of this type consists of an assembly of flat plates 35between which corrugated plates 36 are inserted in order to maintainmechanical strength and improve the transfer of heat.

These plates define channels 37 through which the fluids taking part inthe heat exchange process flow.

The gas to be treated and fed into the exchanger via the line 2circulates through the channels 37 in an upward direction and isgradually cooled by the heat exchange medium. The interleaved corrugatedplates 36 fulfilling the role of a structured lining promote the contactbetween the rising gas and the liquid fractions moving downwards. Thesolvent fed in via the line 3 is evenly distributed cross the top of thechannels 37 in which the gas to be treated is flowing.

If a self-cooling process is used, the dehydrated gas is discharged fromthe head of the exchanger via the line 6, cooled by expansion using amethod such as that described in relation to FIG. 3, for example, andfed back into the upper part of the exchanger by means of the passage 38which provides a discharge essentially perpendicular to the plane of thesection illustrated in FIG. 11 and links up with feeder area for thechannels which is not illustrated in the drawing. It is then evacuatedafter thermal exchange by means of conduct 39, perpendicular to theplane of the section illustrated in FIG. 11. The delivery and dischargemeans are devices well known to the skilled person and provide a passagefor the fluids flowing from each of channels into the discharge pipe andconversely distribute the fluid from one passage into the differentchannels.

The liquid hydrocarbon phase, which may have been stabilised by thereboiler B₃, is discharged at a controlled level via the line 7.

Other types of plate exchangers may also be used, such as exchangerswith stainless steel plates welded to one another, either edge to edgeor across their entire surface, using a diffusion welding technique, forexample.

The plates are made by a blasting process, for example, or by chemicalengraving.

Clearly, the skilled person could use any of the known and availabletechniques to improve the contact between the phases and/or the fluiddistribution without departing from the scope of the invention.

FIG. 12 illustrates an embodiment of a plate which allows phases to bedrawn off depending on their nature, using a process such as thatdescribed in relation to FIG. 4, for example.

The plate has ducts 40 which allow the gas to rise towards the upperpart of the exchanger. The liquid phase which is collected on this platecan be discharged via the passage 15 at a controlled flow rate but mayalso be allowed to overflow down to the lower part of the exchanger. Itis therefore possible to set the system up so that only a fraction ofthe liquid phase arriving from the upper part of the exchanger iscollected.

If two liquid phases are drawn off from the plate, a liquid hydrocarbonphase and an aqueous phase for example, they can be dischargedseparately at least to a certain extent. The aqueous phase, which is theheaviest, tends to collect at the base of the plate and it is possibleto discharge it through perforations 41, for example, provided in theplate.

Another method of discharge known to the skilled person could be usedfor one or the other of the phases without departing from the scope ofthe invention.

Generally speaking, any heat exchange technology can be used providedthat it allows:

the process of heat exchange to be effected in counter-flow,

separate circulation of several fluids which can then be delivered anddischarged separately.

A contact and exchange zone Zi must be capable of operating asillustrated in the diagram of FIG. 13, providing a passagesimultaneously for:

one or more gaseous fractions arriving via the line 70, a liquid phasegenerated by cooling in the zone Zi circulating in counter-flow,producing a gaseous fraction which is discharged from the head of thezone Zi via the passage 71 and a liquid fraction which is dischargedfrom the base of zone Zi via the passage 72,

one or more liquid fractions arriving via the passage 80 which maycontain a solvent, a vapour phase generated by heating in the zone Zicirculating in counter-flow, producing a gaseous fraction which isdischarged from the head of zone Zi via the passage 81 and a liquidfraction which is discharged from the base of zone Zi via the passage82,

one or more fluids participating in the cooling or heating process inzone Zi, whereby a coolant, for example, can be delivered via thepassage 91 at the head of zone Zi and discharged via the passage 92 atthe base of zone Zi.

We claim:
 1. A method of treating natural gas containing at least waterin order to remove at least part of the water from said gas,characterized in that it comprises at least the following steps:a)feeding the natural gas to be treated by means of a first line andfeeding a liquid fraction containing at least an aqueous phase via asecond line, in the presence of a solvent, into a contact zone so as tobring said gas into direct contact with said liquid fraction over atleast a portion of the contact zone, said solvent being anon-hydrocarbon compound other than water, and simultaneously at leastsome of said gas is cooled in the presence of said solvent in order tocondense at least one liquid phase essentially consisting of water in amixture with said solvent, so as to produce at least asolvent-containing phase, an aqueous phase and different condensiblecomponents initially present in the gas to be treated, and b) separatingnon-condensed gaseous phase condensed liquid phase, and one or morecondensible components.
 2. A method as claimed in claim 1, characterizedin that the said liquid phase containing the solvent is delivered to theupper part of the zone in which the gas and liquid fraction are incontact.
 3. A method as claimed in claim 1, characterized in that themethod used to bring the solvent phase and the gas to be treated incontact is a system of counter-flow between the rising gas anddownwardly moving liquid phase.
 4. A method as claimed in claim 1,characterized in that at least some of the gas to be treated is cooledby producing a temperature gradient which varies as a function of thenature of at least one of said gas or the nature of said solvent.
 5. Amethod as claimed in claim 1, characterized in that the cooling processis effected using at least some of the treated gas.
 6. A method asclaimed in claim 1, characterized in that at least one of temperature ortemperature gradient is determined during processing and the quantity ofsolvent phase to be injected is determined on the basis of the nature ofthe composition of said solvent phase and the operating conditions.
 7. Amethod as claimed in claim 1, characterized in that the solvent isselected from the group consisting of methylpropylether,ethylpropylether, dipropylether, methyltertiobutylether,dimethoxymethane, dimethoxyethane, ethanol, methoxyethanol, andpropanol.
 8. A method as claimed in claim 1, characterized in that thesolvent is methanol.
 9. A method as claimed in claim 1, characterized inthat steps a) and b) are carried out on a gas containing at least waterand constituents other than water which are condensible at differenttemperatures, by placing the gas in contact with an aqueous phasecontaining a liquid phase during step (a) and said liquid phase fromwhich at least some of the solvent has been essentially removed isseparated into an aqueous phase and one or more phases containing saidcondensible constituents.
 10. A method as claimed in claim 9,characterized in that a natural gas containing at least methane, waterand high hydrocarbons from C₂ to C₅ (LPG fraction) is treated, and thetreated gas and at least one of the water the LPG fraction or gasolineare separated.
 11. A plant for treating a gas containing at least waterin order to remove at least some of the water from the gas to betreated, characterized in that it comprises an enclosure provided with acooling circuit, a delivery line for said gas to be treated and adelivery line for at least one liquid phase containing at least onesolvent, each of the said delivery lines being linked to a main circuitallowing said gas and said solvent phase to be brought into contact, atleast one discharge line for treated gas and at least one discharge linefor a liquid phase, at least means for recovering separated componentsinside the plant and discharge lines for discharging the separatedcomponents, said discharge lines being connected to said main contactcircuit and said cooling circuit which communicates thermally with saidmain contact circuit.
 12. A plant as claimed in claim 11, characterizedin that the said delivery line for the said liquid phase containing thesolvent is located at the upper part of the enclosure.
 13. A plant fortreating a gas containing at least water and constituents which arecondensible at different temperatures as claimed in claim 12,characterized in that it has at least one means for recovering the saidcondensed constituents and at least one discharge line communicatingwith the said recovery means.
 14. A plant as claimed in claim 1,characterized in that it has at least one means for stabilizing the saidcondensed constituents linked to one of the said discharge lines for thecondensed constituents.
 15. A process of dehydrating a natural gascontaining water and at least one hydrocarbon other than methane andseparating at least some of the at least one hydrocarbon other thanmethane, characterized in that it comprises at least the followingsteps:a) feeding the gas to be treated by means of a first line andfeeding a liquid fraction containing at least an aqueous phase via asecond line, in the presence of a solvent, into a contact zone so as tobring said gas into direct contact with said liquid fraction over atleast a portion of the contact zone, said solvent being anon-hydrocarbon compound other than water, and at least some of said gasis cooled in the presence of the solvent in order to condense at leastone liquid phase essentially consisting of water in a mixture with thesolvent, and b) separating non-condensed gaseous chase from condensedliquid phase from which the solvent has essentially been removed.
 16. Amethod as claimed in claim 1, characterized in that the solvent isselected from the group consisting of alcohols, amines and ketones. 17.A plant according to claim 11, wherein said plant further comprises anexchanger for cooling the gas to be treated prior to feeding said gas tobe treated into the enclosure, and wherein exchanger cools with at leastone of water or air.
 18. A plant for treating a gas containing at leastwater in order to remove at least some of the water from said gas,comprising an enclosure including(a) a delivery line for said gas to betreated and a delivery line for at least one liquid phase containing atleast one solvent, each of said delivery lines being linked to a maincircuit allowing said gas and said solvent phase to be brought intocontact, (b) a cooling circuit which communicates thermally with saidmain contact circuit, (c) at least one discharge line for treated gasand at least one discharge line for a liquid phase, (d) at least meansfor recovering separated components inside the plant and lines fordischarging the separated components, wherein said means for recoveringseparated components allow for selective recovery of condensiblecomponents, and (e) said discharge lines being connected to said maincontact circuit and a cooling circuit which communicates thermally withsaid main contact circuit.
 19. A plant for treating a gas as in claim18, wherein said means for recovering separated components allowing forselective recovery of condensible components comprise at least onerecovery plate for condensed components and discharge lines incommunication said recovery plates.
 20. A plant according to claim 11,wherein said enclosure further comprises a temperature controller.